How to mix shaders depending on the direction normals are facing?
Try using the Layer weight node with the Facing value as a mix factor:



The Layer weight node gives a map based on the angle of the surface to the camera:



When used to mix a diffuse and a transparent shader:



Blending:

The Blend value controls the blending between angles:

With a low Blend value:



With a high Blend value:



As mentioned by CharlesL you could further control the blending with a Color Ramp node: